AT393359B - METHOD FOR PREVENTING THE FORMATION OF PCDD AND / OR PCDE IN COMBUSTION OR. GASIFICATION PLANTS - Google Patents

Description

AT 393 359 B

The invention relates to a method for preventing the formation of PCDD and / or PCDF in incineration or gasification plants, in particular waste incineration plants and coal firing or gasification, in which inhibitors for the catalytic recombination of PCDD and / or in front of a filter for the exhaust gases. or PCDF, such as preferably tert-aliphatic amines, in particular triethanolamine, can be fed into the gas stream.

Chlorinated dibenzodioxins and furans are known as toxic substances, and the formation or formation of such toxic substances from a number of starting materials can be carried out using high-temperature processes, such as combustion and gasification processes. Any chlorinated dibenzodioxins and furans contained in the raw materials to be burned or gasified are decomposed at the combustion or gasification temperatures. However, since the toxic compounds mentioned are extremely stable compounds, they can easily be regenerated by recombination in the presence of carbon compounds and chlorine during cooling, the formation of such compounds being significantly accelerated by catalytically active substances in the exhaust gases. Such catalytically active substances, such as heavy metals, can easily be contained in the exhaust gases when domestic and industrial waste is burned, and the combustion of chlorine-containing coal in combustion plants, in particular power plant combustion plants, can lead to the undesirable formation of such chlorinated dibenzodioxins or furans .

A number of measures have already been proposed for the removal of chlorinated dibenzodioxins and furans, all of which aim to separate the already formed chlorinated dibenzodioxins and furans from the exhaust gas by filtering, adsorption or the like. However, such secondary measures subsequently lead to a highly toxic concentrate of such chlorinated dibenzodioxins and furans. Therefore, ways were sought to prevent the formation or formation of such chlorinated dibenzodioxins and furans in waste incineration or processes in which such a formation is to be feared.

From UWSF-Z. Environmental chemistry Ecotox. (1989) 4: pp. 3-6, it has already become known to use inhibitors which prevent the generation of these pollutants when incinerating waste. The inhibitors which have become known essentially comprise tertiary aliphatic amines, in particular triethanolamine and triethylamine, and mixtures of such amines, it having been proposed to inject these inhibitors into the flue gas at temperatures below the decomposition temperature of the inhibitors, in particular at temperatures of approximately 400.degree. According to the cited literature reference, the use of such inhibitors has led to a reduction in the PCDD and / or PCDF load by more than 95%, the use of such inhibitors and the effect of these inhibitors essentially confirming that substantial parts of the pollutant loads produced in domestic waste incineration plants are involved PCDD and PCDF only emerge after the actual combustion, namely when the flue gases are cooled in the boiler for the purpose of generating energy.

In UWSF-Z. Environmental chemistry Ecotox. (1989) 4: pp. 3-6 further attempts are reported according to which lime was added to incorporate the chlorine content in order to support the action of the inhibitor. However, it has been found that the inhibitor acts without adding lime to the fuel.

The use of lime to incorporate the chlorine content and thus to separate an essential reaction partner for the formation of chlorinated dibenzodioxins and furans is also independent of the fact that they showed no improvements in connection with the inhibitors used, which is not desirable for other reasons in waste incineration plants In which flue gases are cooled for the purpose of energy generation, calcium compounds are deposited on the cooling surfaces and lead to an increase in the cooling channels with a simultaneous substantial reduction in the efficiency of the cooling. Calcium compounds are usually already contained to a considerable extent in the materials used in the incineration of waste and such calcium contents lead to an increase in the amount of slag in the incineration. The proportion of such calcium compounds entering the exhaust gas path is largely converted to anhydrite, a significant reaction with chlorine which may be present in the exhaust gas not being observed. In addition, the calcium oxide contained in the exhaust gas would react quickly to calcium carbonate due to the excess supply of CO2, so that no significant binding of chlorine is to be expected here either.

The invention now aims to develop a method of the type mentioned in such a way that the pollutant load, in particular the load of PCDD and / or PCDF is further reduced and at the same time an increase in undesired deposits on superheaters or cooling surfaces in the exhaust gas path is suppressed. To achieve this object, the method according to the invention essentially consists in that MgO and / or Mg (OH) 2 and / or MgCO ^ is blown into the gas stream before the inhibitors are fed in.

The fact that before the inhibitors MgO and / or Mg (OH) 2 and / or MgCOj is injected into the gas stream ensures that any chlorine contained in the gas stream is safely bound by magnesium, which increases the pollutant content, particularly the load PCDD and / or PCDF can be further reduced. The feeding of such magnesium compounds into the gas stream has the additional advantage that MgO does not slag but actually takes effect in the gas stream and also the advantage that any deposits formed in the starting material to be burned -2-

Calcium compounds contained in AT 393 359 B can be plasticized and blasted off the cooling surfaces. Overall, the addition of such magnesium compounds in the gas stream not only leads to a reduction in pollutant loads but also to an improvement in the efficiency of heat recovery, since an increase in deposits and an increase in the flow paths is prevented. Magnesium compounds of the type mentioned are characterized by the fact that they react quickly with halogens even at relatively high temperatures and that this reaction with halogens is not suppressed by the excess supply of CC> 2 and SO2. The use of Mg (OH> 2 leads to a rapid decomposition of the blown-in magnesium compounds with the release of water vapor immediately after the feed, the MgO formed here having statu nascendi having a particularly high reactivity towards halogen compounds and halogens. Magnesium salts of the type mentioned, after caustification in particular, they are also outstandingly suitable as adsorbents, which preferentially bind a number of other pollutants to the surface of the MgO particles formed or the MgO particles introduced.MgO is distinguished from calcium oxide by a substantially greater porosity and thus a much larger one specific surface, which at the same time ensures that the blown-in particles are easily kept in suspension over the entire exhaust gas path and can subsequently be retained in the filter - even the addition of relatively small amounts of such magnesium Compounds leads, as has been shown in practice, to a significant decrease in the deposits on superheaters or cooling surfaces and the fact that catalytically active substances, such as heavy metals, are adsorbed to a high degree on the surface in such a way: thermally highly resistant magnesium compounds as a result, when inhibitors are injected, the inhibition of the catalytic formation of chlorinated dibenzodioxins and furans occurs almost quantitatively. When Mg (OH) 2 and MgCOj are used, MgO is formed by thermal decomposition and the thermal decomposition leads directly to a significant one Reduction of the maximum grain size of the magnesium compounds fed in. For this reason, the limitation of the grain size is not particularly critical when using Mg (OH) 2 or MgCOß, since correspondingly active particles with a high effective surface are formed immediately during thermal decomposition. Only when MgO is used, as is the case with a preferred method of carrying out the process, is the procedure followed by blowing in MgO with a maximum particle size of 100 μm, preferably 50 pm. This ensures that a large reaction area for the reaction with halogen »! and for the adsorption of other pollutants or catalytically active substances »! is available.

The process according to the invention is preferably carried out in such a way that MgO and / or Mg (OH> 2 and / or MgCOj is blown in at a point on the exhaust gas path at which the exhaust gas temperatures are at least 600 ° C., preferably at least 800 ° C. The fact that the magnesium compounds mentioned are blown in at exhaust gas temperatures of at least 600 ° C., preferably at least 800 ° C., ensures, on the one hand, that the magnesium compounds remain in contact with the gas for a sufficient period of time until the gas passes the filter On the other hand, it ensures that when Mg (OH) 2 and MgCOß are used, causticization occurs quickly enough. When using MgCOg, the process can even be controlled at temperatures of 800 ° C so that causticization does not occur immediately and that most of the implementation takes place when past deposits have already been deposited on superheaters or cooling surfaces Mixing decomposing kinetic energy leads to a rapid detachment of deposits that may have already formed and thus increases the service life of the superheaters or coolers.

In a particularly simple manner, the method according to the invention can be carried out in such a way that MgO and / or Mg (OH) 2 and / or MgCO ^ with secondary air is made above the combustion level, thereby ensuring at the same time that the feed at the required minimum temperatures of 600 ° C or 800 ° C

The MgO used can be formed by appropriate process control in such a way that it has a high specific surface. In caustic processes, the reactivity of the magnesium oxide used can be increased by latent lattice strain, the extent of such latent lattice strain from the process control during manufacture of the caustic product. In particular, if the process is carried out appropriately, porosities of the order of 50% can be achieved, and a magnesium oxide with a specific surface area of > 5 m ^ / g preferably > 10 m ^ / g used. Such a magnesium oxide leads to an almost quantitative immobilization of heavy metals, a corresponding excess of magnesium compounds having to be used in order to be able to bind sulfur compounds safely at the same time. The extent of the magnesium compounds to be introduced can be checked and subjected to regulation in a particularly simple manner on account of the relatively rapid and essentially quantitative conversions when using magnesium compounds, the preferred procedure being that the amount of Mg-V compounds introduced is dependent on the pH -Value of an »aqueous slurry of deposits occurring in the gas path is regulated. In particular, such a regulation should be carried out in such a way that the amount of Mg compounds to be introduced is increased if the pH falls below 8. -3-

AT 393 359 B

Setting such a pH value of over 8 ensures that sulfur compounds contained in the exhaust gases are also set securely and that sufficient reactivity remains for the setting of halogens or halogen compounds. At the same time, corrosion protection is achieved.

The known inhibition, in which heavy metals present in the exhaust gas are complexed, as a result of which they are no longer effective for a catalytic reaction, noticeably reduces the amount of PCDD and / or PCDF which is regressed. The only requirement placed on the inhibitors used is that they do not decompose themselves at the temperature of the reaction with the heavy metals, which has proven particularly advantageous when using triethanolamine and triethylamine mixtures in the order of 400 ° C. In principle, the method according to the invention, in which magnesium compounds are used for setting and eliminating the halogens, can be used with any inhibitors that have a corresponding temperature stability. In this sense, any corresponding temperature-stable catalyst poison should be regarded as an inhibitor.

Overall, the use of the magnesium compounds proposed according to the invention together with inhibitors of this type leads to a further reduction in the amount of pollutant emitted or the amount of pollutant formed, and at the same time significant advantages in process control are also achieved when waste with a high calcium salt content is used

In the course of the process management according to the invention, it has been found in conventional household, commercial or Municipal waste has been proven to be sufficient to use magnesium compounds in an amount of 200 g / t to 2500 g / t waste. Only when special waste is incinerated may it be necessary to further increase the proportion of magnesium compounds to reduce the formation of PCDD / F.

Blowing in magnesium compounds at exhaust gas temperatures of at least 600 ° C is not only advantageous with regard to the causticization of compounds such as Mg (OH) 2 and MgCO ^ still required. Even with the use of MgO, at such a temperature there is an optimum of rapid reaction with halogens and thus a quantitative separation, the MgC ^ formed having proven to be sufficiently stable and no noteworthy tendencies towards renewed decomposition or salting up to the filter revealed. Compared to calcium oxide, magnesium oxide not only has the advantage that the extent of deposits on the superheaters is significantly reduced and even deposits that have already formed are detached, but also the advantage that MgO has no appreciable tendency to form new MgCO ^ at temperatures below 800 ° C, although the (X ^ -partial pressure in the exhaust gas is almost certainly greater than the partial pressure of all other gas components. CaO would also rapidly form CaCO at temperatures below 800 ° C, especially at temperatures between 600 ° C and 700 ° C ^ and thus lead to an even faster growth of deposits on cooling surfaces or superheaters.

The invention is explained in more detail below with the aid of an embodiment shown schematically in the drawing. This shows schematically a waste incineration plant in which the additives provided according to the invention, namely inhibitor and MgO and / or MgC03 and / or Mg (OH) 2 in the

Waste incineration path can be introduced.

In this case, garbage to be burned, in particular commercial or domestic waste, is applied to an inclined grate (2) via a feed pipe (1), on which the garbage is incinerated. 3) applied and subsequently held

The combustion of the garbage is controlled in this system so that nozzles (6) for introducing secondary air as well as those provided according to the invention, the new formation of PCDD and PCDF in a flue (4) in a plane above the combustion zone (5) of the garbage Preventing surcharges is provided 45 Hiebei are fed through a line (7) together with the secondary air MgO and / or MgCO ^ and / or Mg (OH) 2 in an amount of 200 to 2500 g / t waste that with secondary air or Exhaust gas enriched with magnesium-containing aggregates is then passed past cooling surfaces (8) into a cooler (9). The exhaust gas »in the cooler (9) reaches temperatures in the order of magnitude of about 400 ° C., whereupon the exhaust gas is passed through line (10) triethanolamine is added as an inhibitor of the recombination of chlorinated dibenzodioxins or 50 dibenzofurans. After leaving the cooler (9), the exhaust gases are passed through a filter (11) in order to remove solid particles from the exhaust gas.

With the aid of such a waste incineration plant, it is possible by adding the additives provided according to the invention, such as MgO and / or Mgöl ^ and / or Mg (OH) 2 and by adding inhibitors

Triethanolamine largely avoid the recombination of polychlorinated dibenzodioxins or dibenzofurans. Up to 98% of the polychlorinated dibenzodioxins and dibenzofurans from the

To separate the waste gas stream from a waste incineration plant. -4- 60

Claims (8)

AT 393 359 B PATENT CLAIMS 1. Process for preventing the formation of PCDD and / or PCDF in incineration or gasification plants, in particular waste incineration plants and coal firing or gasification, in which inhibitors for the catalytic recombination of PCDD and / or PCDF, such as preferably tert. Aliphatic amines, in particular triethanolamine, can be fed into the gas stream, characterized in that MgO and / or Mg (OH) 2 and / or MgCO ^ is blown into the gas stream before the inhibitors are fed. 2. The method according to claim 1, characterized in that MgO is blown with a maximum grain size of 100 fim, preferably 50 μσι. 3. The method according to claim 1 or 2, characterized in that MgO and / or Mg (OH> 2 and / or MgCOß is blown in at a point on the exhaust gas path at which the exhaust gas temperatures are at least 600 ° C, preferably at least 800 ° C. 4. The method according to claim 1, 2 or 3, characterized in that MgO and / or Mg (OH> 2 and / or MgC03 is introduced with the secondary air above the combustion level. 5. The method according to any one of claims 1 to 4, characterized in that MgO with a specific surface area of > 5 m ^ / g preferably > 10 m ^ / g is used. 6. The method according to any one of claims 1 to 5, characterized in that the amount of Mg compounds introduced is controlled depending on the pH of an aqueous slurry of deposits occurring in the gas path. 7. The method according to claim 6, characterized in that when the pH falls below 8, the amount of Mg compounds to be introduced is increased. 8. The method according to any one of claims 1 to 7, characterized in that Mg compounds are used in an amount of 200 g to 2500 g / t MüU. For this 1 sheet drawing -5-